The development of radiation detectors in the field of nuclear and particle physics has had a terrific impact in medical imaging since this latter discipline took off in late ’70 with the invention of the CT scanners. The massive use in High Energy Physics of position sensitive gas detectors, of high Z and high density scintillators coupled to Photomultiplier (PMT) and Position Sensitive Photomultipliers (PSPMT), and of solid state detectors has triggered during the last 30 years a series of novel applications in Medical Imaging with ionizing radiation. The accelerated scientific progression in genetics and molecular biology has finally generated what it is now called Molecular Imaging. This field of research presents additional challenges not only in the technology of radiation detector, but more and more in the ASIC electronics, fast digital readout and parallel software. In this series of three lectures I will try to present how high energy physics and medical imaging development have both benefited by the cross-fertilization of research activities between the two fields and how much they will take advantage in the future. With particular evidence to Medical Imaging I will address and discuss: 1 - The use of gas, scintillator and solid state detectors in digital radiology and digital mammography, clinical CT and small animal CT. 2 - The use of scintillators and PMT/PSPMTs in functional imaging and in particular: - For Molecular Imaging with PET and SPECT (clinical and preclinical); - For Breast Cancer Imaging (PEM and SPEM, PEMT and SPEMT); - For on line PET dosimetry in hadrontherapy. 3 - The impact of the novel solid state photomultipliers in Medical Imaging and the advent of multimodality imaging such as PET/CT and SPECT/CT, PET/MRI and SPECT/MRI.